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1.
High-temperature reactions were investigated in the MoO3-Ag2O system by means of X-ray, DTA and scanning microscopy methods, and a model was proposed according to which first an Ag2Mo4O13 phase was formed at the MoO3 and Ag2O (or metallic Ag) interface. Subsequently, at the Ag2Mo4O13-Ag2O contact area a layer of Ag2Mo2O7 appeared. If the amount of silver oxide is sufficiently high, a layer of Ag2MoO4 is formed between the Ag2O and Ag2Mo2O7 phases. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

2.
The solid-solid interactions between manganese and magnesium oxides in absence and in presence of small amounts of Li2O have been investigated. The molar ratios between manganese and magnesium oxides in the form of Mn2O3 and MgO were varied between 0.05:1 to 0.5:1. The mixed solids were calcined in air at 400-1000°C. The techniques employed were DTA, XRD and H2O2 decomposition at 20-40°C.The results obtained revealed that solid-solid interactions took place between the reacting solids at 600-1000°C yielding magnesium manganates (Mg2MnO4, Mg6MnO8, MgMnO4 besides unreacted portions of MgO, Mn2O3 and Mn3O4). Li2O-doping (0.75-6 mol%) of the investigated system followed by calcination at 600 and 800°C decreased progressively the intensity of the diffraction lines of Mn2O3 (Bixbyite) with subsequent increase in the lattice parameter 'a' of MgO to an extent proportional to the amount of Li2O added. This finding might suggest that the doping process enhanced the dissolution of Mn2O3 in MgO forming solid solution. This treatment led also to the formation of Li2MnO3. Furthermore, the doping with 3 and 6 mol% Li2O conducted at 800°C resulted in the conversion of Mn2O3 into Mn3O4, a process that took place at 1000°C in absence of Li2O. The produced Li2MnO3 phase remained stable by heating at up to 1000°C. Furthermore, Li2O doping of the investigated system at 400-1000°C resulted in a progressive measurable increase in the particle size of MgO.The catalytic activity measurements showed that the increase in the molar ratio of Mn2O3 in the samples precalcined at 400-800°C was accompanied by a significant increase in the catalytic activity of the treated solids. The maximum increase in the catalytic activity expressed as reaction rate constant measured at 20°C (k 20°C) attained 3.14, 2.67 and 3.25-fold for the solids precalcined at 400, 600 and 800°C, respectively. Li2O-doping of the samples having the formula 0.1 Mn2O3/MgO conducted at 400-600°C brought a progressive significant increase in its catalytic activity. The maximum increase in the value of k 20°C due to Li2O attained 1.93 and 2.75-fold for the samples preheated at 400 and 600°C, respectively and opposite effect was found for the doped samples preheated at 800°C.This revised version was published online in November 2005 with corrections to the Cover Date.  相似文献   

3.
Single crystals of K3RESi2O7 (RE=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were grown from a potassium fluoride flux. Two different structure types were found for this series. Silicates containing the larger rare earths, RE=Gd, Tb, Dy, Ho, Er, Tm, Yb crystallize in a structure K3RESi2O7 that contains the rare-earth cation in both a slightly distorted octahedral and an ideal trigonal prismatic coordination environment, while in K3LuSi2O7, containing the smallest of the rare earths, lutetium is found solely in an octahedral coordination environment. The structure of K3LuSi2O7 crystallizes in space group P63/mmc with a=5.71160(10) Å and c=13.8883(6) Å. The structures containing the remaining rare earths crystallize in the space group P63/mcm with the lattice parameters of a=9.9359(2) Å, c=14.4295(4) Å, (K3GdSi2O7); a=9.88730(10) Å, c=14.3856(3) Å, (K3TbSi2O7); a=9.8673(2) Å, c=14.3572(4) Å, (K3DySi2O7); a=9.8408(3) Å, c=14.3206(6) Å, (K3HoSi2O7); a=9.82120(10) Å, c=14.2986(2) Å, (K3ErSi2O7); a=9.80200(10) Å, c=14.2863(4) Å, (K3TmSi2O7); a=9.78190(10) Å, c=14.2401(3) Å, (K3YbSi2O7). The optical properties of the silicates were investigated and K3TbSi2O7 was found to fluoresce in the visible.  相似文献   

4.
The orientation relationships of Ti-rich barium titanate phases formed by solid state reactions at high temperatures were studied using (110) TiO2 (rutile) and (001) BaTiO3 single-crystal substrates. Well-oriented Ba6Ti17O40 islands were observed after a vapor–solid reaction of a BaO quantity equivalent to a nominal BaO film thickness of 1 nm with the TiO2 substrate, whereas a thin film consisting of well-oriented BaTiO3 and Ba6Ti17O40 grains was formed after vapor–solid reaction of a BaO quantity equivalent to a nominal BaO film thickness of 50 nm with the rutile substrate. A topotaxial orientation relationship between Ba6Ti17O40 and TiO2 was found. Topotaxy is facilitated by a certain similarity in the oxygen sublattices of TiO2 and Ba6Ti17O40. The mechanism of the reaction occurring between BaO vapor and the TiO2 surface at high temperature is discussed. On the other hand, several well-oriented Ba4Ti13O30, Ba6Ti17O40 and Ba2Ti5O12 phases were observed to be embedded in the mainly forming Ba2TiSi2O8 phase after a solid–solid reaction of amorphous SiO2 thin films with (001) BaTiO3 substrates at temperatures above 1000 °C. They were formed by a topotaxial reaction involving the transformation of (111) planes of BaTiO3 into (001) planes of the Ti-rich phases by removal of BaO and insertion of TiO2. Cross-sections of the interfaces between the substrates and the various reaction products are studied by (high-resolution) transmission electron microscopy.  相似文献   

5.
Gases released during the conversion of NH4Zr2(PO4)3 to HZr2(PO4)3 were identified using an apparatus in which gases released from a sample placed in a thermogravimetric analyzer were directly introduced to a gas cell of an IR spectrometer. Such acidic gases as N2O and NO were detected besides the basic NH3 gas, and their formation mechanism was discussed.This revised version was published online in November 2005 with corrections to the Cover Date.  相似文献   

6.
The phase relations in the system In2O3–TiO2–MgO at 1100 and 1350°C are determined by a classical quenching method. In this system, there are four pseudobinary compounds, In2TiO5, MgTi2O5 (pseudobrookite type), MgTiO3 (ilmenite type), and Mg2TiO4 (spinel type) at 1100°C. At 1350°C, in addition to these compounds there exist a spinel-type solid solution Mg2−xIn2xTi1−xO4 (0≤x≤1) and a compound In6Ti6MgO22 with lattice constants a=5.9236(7) Å, b=3.3862(4) Å, c=6.3609(7) Å, β=108.15(1)°, and q=0.369, which is isostructural with the monoclinic In3Ti2FeO10 in the system In2O3–TiO2–MgO. The relation between the lattice constants of the spinel phase and the composition nearly satisfies Vegard's law. In6Ti6MgO22 extends a solid solution range to In20Ti17Mg3O67 with lattice constants of a=5.9230(5) Å, b=3.3823(3) Å, c=6.3698(6) Å, β=108.10(5)°, and q=0.360. The distributions of constituent cations in the solid solutions are discussed in terms of their ionic radius and site preference effect.  相似文献   

7.
Microstructures of three Bi-W-Nb-O phases have been examined by using high-resolution transmission electron microscopy. Bi17W2Nb3O39 and Bi17WNb3O36 have incommensurate superstructures derived from the defect fluorite-type δ-Bi2O3 and can be regarded as intermediate phases between the type II solid solutions in the Bi-Nb-O and Bi-W-O systems. Bi8W2Nb2O23 has a Bi2WO6-like subunit cell with a stepped superstructure. Formation mechanisms of various superstructures are discussed.  相似文献   

8.
This paper examines the structural changes with temperature and composition in the Yb2Si2O7-Y2Si2O7 system; members of this system are expected to form in the intergranular region of Si3N4 and SiC structural ceramics when sintered with the aid of Yb2O3 and Y2O3 mixtures. A set of different compositions have been synthesised using the sol-gel method to obtain a xerogel, which has been calcined at temperatures between 1300 and 1650 °C during different times. Isotherms at 1300 and 1600 °C have been analysed in detail to evaluate the solid solubility of Yb2Si2O7 in β-Y2Si2O7 and γ-Y2Si2O7. Although Yb2Si2O7 shows a unique stable polymorph (β), Yb3+ is able to replace Y3+ in γ-Y2Si2O7 and δ-Y2Si2O7 at high temperatures and low Yb contents. IR results confirm the total solid solubility in the system and suggest a constant SiOSi angle of 180° in the Si2O7 unit across the system. The temperature-composition diagram of the system, obtained from powder XRD data, is dominated by the β-RE2Si2O7 polymorph, with γ-RE2Si2O7 and δ-RE2Si2O7 showing reduced stability fields. The diagram is in accordance with Felsche's diagram if average ionic radii are assumed for the members of the solid solution at any temperature, as long as the β-γ phase boundary is slightly shifted towards higher radii.  相似文献   

9.
YBa2Cu3Ox (Y-123) and Bi2Sr2Ca1Cu2Ox (Bi-2212) films on various substrates have been prepared by Metal-Organic Deposition starting from different metallorganic fluorine-free compounds and using a very simple instrumentation. The processing conditions include a rapid pyrolysis step in air and an annealing step in oxygen for Y-123 and in air for Bi-2212. The films obtained have been characterized by X-ray diffraction (XRD) and the formation of a superconducting phase of Y-123 or Bi-2212 was confirmed measuring the critical temperature (T c) with Ac-susceptibility and resistive measurements. Microstructure and final cationic ratios have been studied by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).  相似文献   

10.
用液相反应-前驱物烧结法制备了Cr2(WO4)3和Cr2(MoO4)3粉体。298~1 073 K的原位粉末X射线衍射数据表明Cr2(WO4)3和Cr2(MoO4)3的晶胞体积随温度的升高而增大, 本征线热膨胀系数分别为(1.274±0.003)×10-6 K-1和(1.612±0.003)×10-6 K-1。用热膨胀仪研究了Cr2(WO4)3和Cr2(MoO4)3在静态空气中298~1 073 K范围内热膨胀行为,即开始表现为正热膨胀,随后在相转变点达到最大值,最后表现为负热膨胀,其负热膨胀系数分别为(-7.033±0.014)×10-6 K-1和(-9.282±0.019)×10-6 K-1。  相似文献   

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